Variability Of Marine Ecosystems Research Articles

Redox geochemical signatures in Mediterranean sapropels: Implications to constrain deoxygenation dynamics in deep-marine settings

Global warming and anthropogenic activity are boosting marine deoxygenation in many regions around the globe. Deoxygenation is a critical ocean stressor with profound implications for marine ecosystems and biogeochemical cycles. Understanding the dynamics and evolution of past deoxygenation events can enhance our knowledge of present-day and future impacts of climate change and anthropogenic pressure on marine environments. Many studies have reconstructed the evolution redox conditions of past deoxygenation events using geochemical proxies. In this regard, the present work focuses on understanding the paleoenvironmental significance of geochemical redox signals derived from the onset, evolution and termination of regional-scale deoxygenations in deep-marine settings, with a specific focus on sapropels in the Eastern Mediterranean (EM). Sapropels, rhythmic organic-rich sediments deposited in EM, offer a unique opportunity to investigate recent deoxygenation events linked to past climate changes. Sapropels serve as paleo-archives of past deoxygenation events and can provide insights into the potential impacts of ongoing climate change on marine ecosystems and biogeochemical cycles. By integrating previous sapropel geochemical studies with a detailed analysis of new geochemical data from five Quaternary sapropels (S1, S5, S6, S7 and S8) in three different EM deep-marine settings, this study enhances our understanding of the paleoenvironmental significance of geochemical redox signals produced by deoxygenation dynamics and postdepositional processes in different deep-marine settings. The study supports that certain trace elements, such as Mo, V, U, Co, and Ni, are identified as more reliable redox proxies compared to Cr, Cu, Pb, and Zn. Four recurrent geochemical intervals attributed to specific redox conditions and postdepositional processes have been identified. Moreover, internal calibration of redox proxies thresholds has been performed and demonstrates how local environmental conditions (e.g., productivity rate) and hydrogeographic features (e.g., water-depth, particulate-shuttling intensity, deep-water renewal and fluvial input) play crucial roles in controlling the authigenic uptake rates of redox-sensitive trace metals, and subsequently, redox thresholds values of geochemical redox proxies. The results also emphasize the importance of postdepositional processes to accurately interpret geochemical signals in paleoenvironmental studies. This research enhances our overall understanding of geochemical signals associated with regional-scale deoxygenation events in deep-marine settings, offering new insights into predicting biogeochemical changes in marine environments undergoing a transition towards anoxia. By comprehending the dynamics of past and present deoxygenation, we acquire valuable knowledge regarding the potential effects of climate variability in marine ecosystems.

Role of Mixed Layer Depth in Kuroshio Extension Decadal Variability

AbstractSea surface temperature (SST) anomalies in the Kuroshio Extension (KE) have been suggested to play a crucial role in decadal climate variability of the North Pacific affecting global climate and marine ecosystem variability. By analyzing the mixed layer heat budget, taking into account seasonality and mixed layer depth (MLD) variations, we here show for the first time that the KE SST front undergoes large decadal variations mainly owing to decadal modulations of the effective heat capacity affecting the SST sensitivity to surface heat fluxes during 1982–2015. More specifically, when the mixed layer becomes anomalously thick (shallow) to the south (north) of the front, it becomes less (more) sensitive to wintertime surface cooling. As a result, the SST front is strengthened, with positive (negative) SST anomalies to the south (north). A heat conservation model suggests that MLD anomalies are mainly due to thermocline depth anomalies.

Science for Good Environmental Status: A European Joint Action to Support Marine Policy

In the last decade, several initiatives have been taken at a European level to adopt the Marine Strategy Framework Directive (MSFD) by promoting coordination and stimulating integrated actions leading to consistent views on its final goal: the achievement of good environmental status (GES). In its holistic approach, the MSFD fully acknowledges the complexity and variability of marine ecosystems and demands constant scientific support for its actual implementation. Recently, the Joint Programming Initiative on “Healthy and Productive Seas and Oceans” (JPI Oceans) launched the joint action “Science for Good Environmental Status” (Science4GES), building on the contribution of different scientific disciplines and communities to better fulfill the scope of the MSFD. In this paper we illustrate and discuss a few crucial aspects of endeavors to implement the MSFD specifically implied in the definition of the metrics for the 11 descriptors and GES in its complexity, as well as improving the strategy governing its implementation. This presentation also describes the challenges, aims and implementation plan for the JPI-O joint action, where a transdisciplinary approach may help in progressing from the comprehensive and far-reaching vision of the MSFD to the achievement of a durable GES.

Impacts of Mesoscale Eddies on Phytoplankton Size Structure

AbstractThe impact of mesoscale eddies on phytoplankton communities attracts considerable research attention because phytoplankton play numerous roles in marine ecosystems. Using remote sensing techniques, this study considered a synoptic relationship between phytoplankton size structure, which is a determinant of energy transfer efficiency in marine ecosystems, and mesoscale eddies, which are ubiquitous ocean features. We found clear spatial variation in the impacts of mesoscale eddies on phytoplankton size structure; phytoplankton communities with larger cell sizes were supported by anticyclonic eddies in oligotrophic regions and by cyclonic eddies in eutrophic regions. Differences in the phytoplankton size structure within these two types of mesoscale eddies became greater, accompanied by interannual trends in the phytoplankton size structure for 2003–2014. Our findings are the first to demonstrate the important linkage between mesoscale eddies and phytoplankton size structure on a global scale; this new knowledge might help better predict variability in marine ecosystems.

Impact of spatiotemporal variability in phytoplankton size structure on benthic macrofaunal distribution in the Pacific Arctic

The size structure of the phytoplankton community strongly affects energy flow through food webs; hence, monitoring the spatial and temporal dynamics of phytoplankton size structure is crucial to evaluate variability in marine ecosystems. In this study, the spatiotemporal changes in phytoplankton size structure and their impact on benthic macrofaunal distribution were examined in the Pacific Arctic as part of the international Distributed Biological Observatory effort. The exponent of the chlorophyll-a (Chla) size distribution (hereafter, CSD slope), which is an index of synoptic size structure of a phytoplankton community, was estimated based on spectral shape of the phytoplankton absorption coefficient using satellite ocean color data. The CSD slope during the post-bloom period was significantly correlated with sediment Chla concentration, suggesting that the amount of organic carbon input to the seafloor was strongly related to surface phytoplankton size structure after the spring bloom. Average macrofaunal biomass in the Pacific Arctic was persistently high (>20 g C m−2) throughout 2000–2013 in the regional hotspots, and the biomass-based latitudinal centroid of macrofaunal habitat in the region shifted significantly (p < 0.05) northward. This northward shift is likely driven by a spatiotemporal change in phytoplankton size structure during the post-bloom period. Our results suggest that bottom-up forcing by the phytoplankton community affected the distribution of benthic macrofauna. We conclude that the remote estimate of phytoplankton size structure largely contributes to advancing our knowledge of ecosystem changes observed in the Pacific Arctic marine ecosystem.

Survey timing vs. ecosystem scheduling: Degree-days to underpin observed interannual variability in marine ecosystems

Fishery annual surveys are shifting towards integrated ecosystem surveys. However, monitoring the whole ecosystem once a year entails special attention to the interpretation of observations, because they only represent a snapshot in the seasonal dynamics of the environment, of a species, or any dynamic process of interest. The dependency of this snapshot to the timing, but also to the duration, of the survey with respect to this seasonal dynamics needs to be considered. Fish stock assessment is only little impacted by the observed timing within a given season, whereas warming, stratification, plankton bloom occurrence, or fish fecundity, are processes rapidly changing especially in spring. Firstly, from independent satellite data, we described the seasonal and interannual Sea Surface Temperature (SST) variability in the Bay of Biscay in spring around the annual PELGAS surveys. Our results revealed that the in-situ surface temperature snapshot from a given survey may be strongly misleading on the existing spatial pattern, blurring the latitudinal temperature gradient, with potential impact in e.g. species habitat modelling. Secondly, based on these survey-independent SST data, we proposed a methodology to position annual surveys in the environment spring schedule. Our temporal indicator is based on a degree-day metric, computed as the cumulated temperature from the previous winter date with minimum annual temperature. The annual degree-day is then compared to the climatology of the degree-day metric to correct the actual date of survey to an environment date. The methodology was spatialised to take into account both latitudinal shift in phasing of seasonality, and rapidly changing conditions in the environment during the survey itself. Finally, we tested the methodology to refine the description of anchovy and sardine spawning phenology, as well as spawning spatial distribution for particular years such as hot year 2003. More generally, this is an example of how synoptic, independent data from operational oceanography, as a background history of the environment dynamics, can be used to improve the interpretation of biological observations in marine ecosystems.

Quantifying and predicting responses to a US West Coast salmon fishery closure

Abstract As anthropogenic changes interact with natural climate cycles, the variability of marine ecosystems is likely to increase. Changes in productivity of particular fisheries might be expected to lead not only to direct impacts within a fishery but to economic and ecological effects on other fisheries if there is substantial cross-participation by fishers. We use data from the US West Coast salmon troll fishery before, during, and after a large-scale closure to illustrate how altered resource availability influences the behaviour of fishing vessels in heterogeneous ways. We find that vessels were less likely to participate in fishing of any type during the closure, with &amp;gt;40% of vessels ceasing fishing temporarily and 17% exiting permanently. Vessels that were more dependent on salmon were more likely to cease fishing while more diversified vessels were more likely to continue. In spite of a high level of cross-participation, we find limited evidence that vessels increased their participation in other fisheries to offset lost salmon revenue. Ports that obtained more of their revenue from salmon troll vessels saw larger decreases in their revenue during the closure. Ocean conditions from 2013 to 2015 suggest the possibility of another highly restricted salmon fishing season in 2017. Our models predict that such restrictions would cause another economic disaster and lead to a large fraction of vessels exiting fishing but suggest that effects on fisheries linked by cross-participation are likely to be low.

Spatio-temporal variability of copepod abundance along the 20 °S monitoring transect in the Northern Benguela upwelling system from 2005 to 2011.

Long-term data sets are essential to understand climate-induced variability in marine ecosystems. This study provides the first comprehensive analysis of longer-term temporal and spatial variations in zooplankton abundance and copepod community structure in the northern Benguela upwelling system from 2005 to 2011. Samples were collected from the upper 200 m along a transect at 20°S perpendicular to the coast of Namibia to 70 nm offshore. Based on seasonal and interannual trends in surface temperature and salinity, three distinct time periods were discernible with stronger upwelling in spring and extensive warm-water intrusions in late summer, thus, high temperature amplitudes, in the years 2005/06 and 2010/11, and less intensive upwelling followed by weaker warm-water intrusions from 2008/09 to 2009/10. Zooplankton abundance reflected these changes with higher numbers in 2005/06 and 2010/11. In contrast, zooplankton density was lower in 2008/09 and 2009/10, when temperature gradients from spring to late summer were less pronounced. Spatially, copepod abundance tended to be highest between 30 and 60 nautical miles off the coast, coinciding with the shelf break and continental slope. The dominant larger calanoid copepods were Calanoides carinatus, Metridia lucens and Nannocalanus minor. On all three scales studied, i.e. spatially from the coast to offshore waters as well as temporally, both seasonally and interannually, maximum zooplankton abundance was not coupled to the coldest temperature regime, and hence strongest upwelling intensity. Pronounced temperature amplitudes, and therefore strong gradients within a year, were apparently important and resulted in higher zooplankton abundance.

Seasonal succession of zooplankton in coastal waters of the Argentine Sea (Southwest Atlantic Ocean): prevalence of classical or microbial food webs

The demographic characteristics of marine zooplankton make it especially suitable for examining the variability of marine ecosystems. The zooplankton annual succession was studied at a permanent coastal station in the Argentine Sea (38°28′S, 57°41′W) in relation to physical conditions and phytoplankton size fractions. Small copepods (<1 mm total length), mainly represented by Oithona nana (Cyclopoida) and adults and copepodites of Calanoida, numerically dominated the metazooplankton throughout the year. In summer, small copepods also exceeded large copepods in biomass. Larvaceans (mostly <1 mm total length) were the second most important metazooplankton group, with strong dominance of Oikopleura dioica. The zooplankton succession exhibited two main periods throughout the year: (1) a cold winter–spring period characterized by a dominant classical herbivore food web in which the large copepod Calanoides carinatus and lamellibranch larvae were associated with the lowest temperatures and highest Chl-a and microphytoplankton, and (2) a warm summer period dominated by a microbial food web in which microbial filter-feeders such as Oithona nana, Paracalanus spp., Oikopleura dioica and Penilia avirostris predominated and the highest density of picophytoplankton and lowest concentrations of Chl-a were recorded. The implications of the present findings for the growth and survival of fish larvae distributed in the study area are discussed.

The emergence of regularity and variability in marine ecosystems: the combined role of physics, chemistry and biology

Marine ecosystems play an integral role in the functioning of life on earth. To predict how they will respond to global changes, and to effectively manage and maintain services upon which humans rely, we must understand how biological processes at the cellular level generate macroscopic patterns in the oceans. Here, we discuss how physics and biogeochemistry influence and constrain marine ecosystem structure and function, and outline key regularities and patterns of variability that models should aim to reproduce. We identify unanswered questions regarding how size-dependent physiological and ecological processes are linked to turbulent mixing, dealing specifically with how size structure is related to mixing over a range of spatial scales and how it is linked to the fate of primary production in the sea.

Linking climate to population variability in marine ecosystems characterized by non-simple dynamics: Conceptual templates and schematic constructs

The ability to abstract and symbolize ideas and knowledge as simplified schematic constructs is an important element of scientific creativity and communication. Availability of such generalized symbolic constructs may be particularly important when addressing a complex adaptive system such as a marine ecosystem. Various examples have appeared in the climate–fisheries literature, each more or less effectively integrating hypothesized effects of several interacting environmental and/or biological processes in controlling population dynamics of exploited fish species. A selection of these are herein presented and reviewed, including match–mismatch, connectivity, school trap, loopholes, ocean triads, stable ocean hypothesis, several classes of nonlinear feedback loops (e.g., ‘ P2P’, school-mix feedback, predator pit), as well as several prominent large-scale integrative climatic index series ( SOI, NAO, PDO). The importance of considering the potential for adaptation and/or rapid evolution is stressed. An argument is offered for the potential utility of such widely recognizable schematic concepts in offering relatively well-understood, fairly well-defined frameworks for comparative identification and elaboration of important mechanistic linkages between climate variability and fishery dynamics, as well as in easing effective communication among scientists from different regions and disciplinary backgrounds. Certain difficulties in the application of the comparative method are discussed. It is suggested that alleviation of such difficulties may be one of the major benefits of international collaborative programs such as GLOBEC and IMBER.

Summing Up - Symposium on Reproductive and Recruitment Processes of Exploited Marine Fish Stocks

Variability in recruitment results in variability in catch from year-to-year, which is a problem for fishermen, for the processing industry and for fisheries managers. It is commonplace to read how large the interannual variability in recruitment can be and how difficult it is to determine the causes. However for an ecologist, the astonishing feature of recruitment variability is how small it is, given that fish produce thousands or millions of eggs each and that survival is extremely unlikely and uncertain. Among the pelagic and demersal stocks assessed in the NE Atlantic the smallest year-class is generally within a factor of 10 of the biggest. It is not infrequent for interannual recruitment variability to be less than the variability of the spawning biomass of the stock (Brander, 2003). How is the variability damped out? What are the compensatory mechanisms? Density dependence is necessary for populations to remain within feasible bounds (Reddingius, 1971) but as Ed told us “A little density dependence, especially in the relatively long juvenile stage, can regulate recruitment”. Density dependence should not be invoked without good evidence in relation to processes like adult growth, however appealing it is to biologists or convenient for modellers trying to keep their models stable. Fisheries assessment and management, at least in the North Atlantic, has focussed on year-to-year tracking of stock biomass in order to set annual catch limits. Questions about the role of recruitment were therefore also principally directed at annual prediction and the understanding of processes needed in order to do this. The biological book-keeping was done using sequential population analysis which gives estimates of both annual recruitment and spawning stock biomass (SSB). The latter is assumed to represent spawning output of the stock, but as many of the papers in this symposium show, fecundity, maturity, spawning frequency and egg quality, are not constant. The relevance of the symposium is therefore obvious, since strategies for sustainable management, using precautionary reference levels of SSB and fishing mortality, are based on the relationship between SSB and recruitment. We need to move on to a longer term view which includes a wider range of information about the biological state of fish, their environment and their interaction with the rest of the marine ecosystem. Ed once more: “’Solving the recruitment problem’ is no longer the holy grail of fishery science. Appreciating recruitment variability, explaining probable causes, considering implications for management, and understanding it in the context of broader variability in marine ecosystems are worthy goals.”

Emerging from Hjort's Shadow

Early in the 20 th century, Johan Hjort developed compelling arguments and hypotheses to explain recruitment variability that became dominant for more than 75 years. A cautious emergence from Hjort’s shadow began late in the 20 th century. Hjort’s “Critical Period” hypothesis, i.e., failure of first-feeding larvae to find food, and a second hypothesis, “Aberrant Drift” of eggs and larvae, were proposed to explain causes of recruitment variability. Tests of the Critical Period hypothesis became an obsession, although support for it was inconsistent and equivocal. Single-minded research on the Critical Period hypothesis gave way to realization that recruitment variability was the outcome of complex trophodynamic and physical processes acting over many temporal and spatial scales and throughout pre-recruit life. A complex mix of main effects and interacting factors can easily generate order-ofmagnitude variability in recruitment via small effects on mortality and growth rates during the abundant egg and larval stages, or via cumulative effects during the pre-recruit juvenile stage. New considerations of compensatory mechanisms that can dampen variability and stabilize recruitment emerged. A little density dependence, especially in the relatively long juvenile stage, can regulate recruitment. Multidisciplinary programs instituted in the 1990s and successful development of statistical models and coupled bio-physical models, offered new insights into mechanisms generating recruitment variability. Despite progress, forecasting recruitment remains a formidable challenge. “Solving the recruitment problem” is no longer the Holy Grail of fishery science. Appreciating recruitment variability, explain ing its probable causes, considering implications for management, and understanding it in the context of broader variability in marine ecosystems, are all worthy goals.

New Tools to Meet New Challenges: Emerging Technologies for Managing Marine Ecosystems for Resilience

ABSTRACT The goal of this article is to highlight evolving tools, recent advances, and emerging techniques that are being used to understand natural variability in marine ecosystems. These technical approaches range from the tagging of large pelagic organisms to the use of genomics to provide insight into the abundance and health of marine organisms. Although these techniques vary dramatically in scale, they share the potential to remove critical impediments to the effective management of marine systems.

Accounting for unresolved spatial variability in marine ecosystems using time lags

The formulation and calibration of models is a vital method for probing and predicting the behavior of marine ecosystems. The ability to do this may suffer, however, if the calibrating data set is subject to significant spatial variability between samples that is not resolved in the model. We propose that some of this variability might be accounted for by variable time lags between sampled water masses which are otherwise assumed to follow a common pattern of ecosystem variability (dynamical trajectory). Using twin tests of fitting models to simulated data sets, we show that realistic levels of meso/sub-mesoscale variability in time lags may have significant distortion effects on the parameter fits from standard methods which do not account for it. The distortion is such as to 'smooth out' or underestimate the magnitude of temporal variability within sampled water masses, causing loss of accuracy and robustness of biological parameter estimates and functions thereof (e.g. gross primary production). A new method of model fitting is shown to avoid these effects, allowing improved estimates over a broad range of spatial time lag variability and measurement noise levels, assuming accurate estimation of the time lag variance, for which we also suggest a method.

Marine ecosystem variability and human community responses: The example of Ghana, West Africa

This study describes variability in the marine ecosystem of Ghana, West Africa, on several temporal and spatial scales and discusses how the human communities using this ecosystem respond to this variability to cope socially and economically. Ghanaian marine waters are part of an upwelling system with strong seasonal and inter-annual variability. Much of this variability is forced at large spatial scales in the tropical Atlantic and by El Niño—Southern Oscillation events in the Pacific Ocean, which influence inter-annual variability of sea surface temperature and pelagic fish landings off Ghana. At decadal scales, Ghanaian marine waters experienced cool sea temperatures and low fishery landings during the 1960s, rapid warming and increases in fishery landings during the late 1970s and 1980s, and variable temperatures and fishery landings during the 1990s. In the late 1990s, pelagic and demersal fish populations appeared to be declining, partly due to over-fishing, although the per capita supply (domestic production plus net imports) of fish was kept high by increased imports. Artisanal fishers and fishing communities in Ghana have devised strategies to deal with variability on seasonal and inter-annual scales. These livelihood strategies include: (i) exploiting marine and terrestrial natural resources more intensively, initially at local scales but expanding to regional scales; (ii) ensuring multiple and diversified income sources; (iii) investing in social relationships and communities for support; and (iv) undertaking seasonal or permanent migrations. In addition, the national government imports fish to deal with shortages. However, these strategies may be less adapted to variability at decadal scales, and may not be sustainable when viewed at the larger scales of environmental change.

Future aspects in marine ecosystem modelling

Existing ecosystem models are briefly presented and summarised. The problem of coupling physical and biological models as well as aspects of prediction and predictability are discussed. The general perception that marine ecosystems are inherently unpredictable due to non-linearity becomes questionable if the response of climate variability in marine ecosystems is analysed. Many authors have shown correlations between climate variability and the variability of abundance or biomass of marine organisms such as phytoplankton, zooplankton, benthos or fish recruitment in different parts of the world ocean. In the northern hemisphere, certain species show a linear response to climate variability mainly during winter and spring. However, the underlying mechanisms are not well understood. Often, a phase lag can be observed between climate variability and the reaction of organisms. The identification of a plausible mediator between climate and biology is difficult, since all possible physicochemical mechanisms having a direct or indirect influence on the variability of abundance or biomass of marine organisms have to be considered as mediator. The understanding of the reason of the phase lag, which possibly implies a “biological memory”, and the identification of all possible mediators are necessary to predict the response of marine organisms to climate variability. The identification of mediators will result in an improvement of coupled models, a deeper understanding of physical–biological interaction and the improvement of predictive capability of marine ecosystem models.

Characterising meso-marine ecosystems of the North Pacific

Abstract To delineate mesoscale variability in marine ecosystems of the subarctic North Pacific and identify “hotspots” of biological activity, we conducted contemporaneous surveys of plankton and avifaunal communites in 2000–2003. Plankton samples were collected with a continuous plankton recorder (CPR) towed by a commercial vessel while a trained observer recorded marine bird distributions using strip-transect techniques. Near- and sub-surface physical oceanographic properties and productivity patterns were measured using a temperature data logger and satellite-derived chlorophyll a concentrations. We identified 10 distinct biological communities across the North Pacific, which we refer to as ‘meso-marine ecosystems’ (MME). We examined the characteristics of MME over multiple years to assess temporal persistence. MME were associated with different bathymetric domains and current systems. MME differed in the overall abundance and species composition of their fauna and, therefore, almost certainly in productivity. Regular monitoring of the spatial and temporal variability of MME will enhance our ability to detect and understand coupled climate-ecosystem responses, and, in turn, help guide ecosystem-based fisheries and wildlife management.